A new modified method for the determination of delta-aminolevulinic acid (ALA) dehydratase activity has been developed. This modified method involves addition of 0.1 mM Zn2+ and then preheating the enzyme solution at 60 degrees C for 5 min before following commonly used procedures. The ALA dehydratase activity in the peripheral blood of lead poisoned rats, determined by this procedure, was approximately 7.5-fold higher than control values. The activity of ALA dehydratase in the bone marrow cells of lead poisoned rats increased approximately 2-fold. These increases were accompanied by a simultaneous decrease in an inhibitor specific for ALA dehydratase. Consequently, these results suggest that ALA dehydratase is induced after the reduction of ALA dehydratase activity, and that ALA dehydratase activity is controlled by a new specific ALA dehydratase inhibitor.